Sun Spot Number , WWV plot
Updated every 0010Z
0904Z = 1804JST SFI=74 SSN=0 A=19 K=4
The A-index provides a daily average level for geomagnetic activity.
Because of the non-linear relationship of the K-scale to magnetometer
fluctuations, it is not meaningful to take averages of a set of K indices.
What is done instead is to convert each K back into a linear scale
called the "equivalent three hourly range" a-index (note the lower case).
The daily A index is merely the average of eight "a" indices.
Thus, for example, if the K indices for the day were 3 4 6 5 3 2 2 1,
the daily A index is the average of the equivalent amplitudes:
A = (15 + 27 + 80 + 48 + 15 + 7 + 7 + 3)/8 = 25.25
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